leavener

Etymology tree
Proto-Indo-European *h₁lengʷʰ-
Proto-Indo-European *-us
Proto-Indo-European *h₁léngʰusder.
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Proto-Italic *breɣʷisinflu.?
Proto-Italic *leɣʷis
Latin levis
Proto-Indo-European *-h₂
Proto-Indo-European *-éh₂
Proto-Indo-European *-yéti
Proto-Indo-European *-eh₂yéti
Proto-Italic *-āō
Latin -ō
Latin levō
Proto-Indo-European *-mn̥
Proto-Italic *-mn̥
Latin -men
Vulgar Latin *levāmender.
Old French levainbor.
Middle English levayn
English leaven
Proto-Indo-European *-yósder.
Proto-Italic *-āzijos
Latin -āriusnom.
Latin -āriusbor.
Proto-Germanic *-ārijaz
Proto-West Germanic *-ārī
Old English -ere
Middle English -ere
English -er
English leavener
From leaven + -er.